1. Field of the Invention
The present invention relates to a liquid, dual curable, adhesive composition capable of polymerization by exposure to actinic radiation and moisture. The composition is particularly useful as liquid adhesives for electronic applications.
2. Description of the Related Art
There is great commercial interest in producing actinic radiation curable adhesive compositions. One-part, ultraviolet (UV) and/or visible light curable, urethane acrylate adhesive compositions are known in the art, however, portions of these compositions remain unreacted and uncured when UV or visible light is blocked and prevented from striking these adhesive compositions. These shadowed areas pose a reliability risk since a less than ideal adhesive bond is formed, and the uncured, wet or tacky adhesive may chemically solubilize or otherwise attack either or both the adjoining cured adhesive areas, or its substrate, and thus weaken the adhesive bond over time. This is an especially difficult problem in optical display assemblies and edgebonding of circuit board components. This invention is directed to a UV or visible light curing adhesive that can be further moisture cured in shadow areas. WO 2012/138675 A1 and WO 2013/013589 A1 describe two-part liquid optically clear adhesive systems which contain an organic peroxide in part A and a peroxide reducing agent in part B. WO 2013/023545 A1 describes a one-part duel cure actinic radiation and heat cured liquid optically clear adhesive. In the described adhesive, a peroxide is used in the formulation that starts a curing reaction in shadow area with heat. These disclosures do not relate to moisture curable adhesives and such are not suitable for temperature sensitive components. US 2012/0329897 A1 describes a one-part actinic radiation and moisture curable liquid composition comprising at least one component (A) with alpha-(alkoxy)silane groups, with the exception of polyolefins substituted with alpha-(alkoxy)silane groups, and at least one radiation-curing component (B) with radically curable groups, where components A and components (B) are present as separate compounds. In this composition, component (B) will stay unreacted in the shadow area because it does not have moisture curable functionality. Component (A) will stay unreacted in the areas where moisture cannot penetrate because it does not have light curable functionality. In other words the moisture curable and UV curable functionalities are not present in the same molecules. U.S. Pat. No. 5,696,179 describes a radiation curable optical glass coating composition which utilizes an oligomer containing both radiation curable functionality and silane functionality. This patent does not relate to a moisture curing formulation in shadow areas. This patent discloses a composition which is targeted to create coatings that will have better moisture resistance, low water absorption and sufficient adhesion to glass. U.S. Pat. No. 6,777,090 describes a one component UV and heat dual curable system. In such a dual cure system the polymerizable molecule contains a primary or secondary carbamate group and at least one bond that can be activated by UV radiation. This system requires heat to cure the carbamate group. PCT publication WO 2013/016136 describes a dual moisture curable system. The material has a part A consisting of an oligomer with UV active polymerizable groups containing isocyanate functionalities and also may contain molecules with polyisocyanate monomers or isocyanate polymers, and a second part B which contains polyol or amine functional crosslinkers. A photoinitiator may be present in either A or B. After UV cure an addition reaction between isocyanate and hydroxyl group of the polyol continues to proceed. The part of the formulation that is not exposed to UV light will be cured by the reaction of the isocyanate and polyol. PCT publication WO2013/013589 describes a two component system having UV as well as activator curing. This two component system is cured by UV and peroxide. In the shadow areas the acrylate functionality is cured by the generation of radicals by the decomposition of peroxides when it comes in contact with an amine type reducing agent and metal based salt such copper salt.
The present invention provides a one part, actinic radiation, i.e. UV and/or visible light and moisture dual curable system utilizing a polyurethane oligomer containing both actinic radiation and moisture curable groups. (Meth)acrylate groups in the oligomer provide crosslinking sites for actinic radiation initiated free radical polymerization, and (alkoxy)silane groups provide crosslinking sites for condensation polymerization in presence of water or moisture. The composition provides a better crosslinked network and a better curing rate because the oligomer contains moisture curable and actinic radiation curable functionality at the same molecule and the formulation does not require separate oligomers for curing with actinic radiation and moisture. The liquid adhesive cures into optically clear materials with both actinic radiation and moisture. Therefore, it is ideal for bonding electronic display parts with shadow areas. Moisture can also be provided by dispensing and admixing water with the composition, or by exposing the composition to water in an atmospheric surrounding the composition. For places where moisture is hard to penetrate, adding water during dispensing accelerates moisture cure reaction significantly.